The line raster on the image in an IR-imaging system, said image being presented by means of the image representation unit and generally on a cathode-ray tube, can be built up of, for example, in two sweeps; one in a horizontal direction (line sweep) and one in a vertical direction (frame sweep). The sweep starts are obtained by trigger signals from an optical-mechanical scanning unit. The scanning may be carried out, for example, by two rotational movements, one horizontal and one vertical. The scanning element consists, for example, of rotating prisms of, for example, silicon, but scanning elements may be used which contain tilting mirrors and rotating reflecting drums. The present invention is applicable to most existing scanning systems, and in the example given in the following a system with rotating scanning units is described.
To obtain the best possible picture of the scanned object, a high scanning frequency is aimed at. The maximum scanning frequency is limited by the mechanical system, the scanning frequency being chosen highest for the horizontal movement. At one revolution of, for example, the line scanning unit the object is scanned a certain number of times depending on the action of the scanning unit. To obtain a good pictorial quality at the photographic recording of the image, high demands are made especially on the precision in the frame trigger system. In photographing it is desirable moreover to have a large number of lines per frame so that the line pattern should not have a disturbing effect on the image. This can be achieved by making use of a so-called interlacing process (sliding line raster), where an image is built up of a certain number of fields which are displaced in relation to one another. The interlacing process raises further the requirement for precision in the trigger system.
In the standard infra-red imaging systems the line trigger signal is produced by the line trigger unit and the frame trigger signal by the frame trigger unit. These trigger units are controlled by the respective scanning unit. As mentioned earlier, the rotational speed of the line scanning unit is greater than the rotational speed of the frame scanning unit, which implies that a relatively good precision can be obtained in the line trigger signal, whilst it is appreciably more difficult to obtain a good precision in the frame trigger signal owing to mechanical imperfections. Seen against this background it will be clear that the demand for precision in the frame trigger system is greater than the demand for precision in the line trigger system, whilst at the same time it is more difficult to achieve a high precision in the frame trigger system.